We plan to study the cellular and molecular mechanisms involved in the differentiation of skeletal muscle. The function and regulation of expression of the muscle-specific membrane protein, H36, will be studied. This protein is a marker of the development of cells at distinct stages in the myogenic lineage. It also distinguishes primary (H36-) from secondary (H364+) myoblasts. H36 cDNA has been cloned: its complete nucleotide and amino acid sequences will determined. Genomic clones will be isolated and the mechanisms by which expression of H36 is regulated at different stages of myogenic development will be studied. We will determine whether the regulatory genes, MyoD1, myogenin, Myf5, Myf6 and Mrf4, can promote expression of H36 in cells at different stages of the myogenic lineage. The functions of H36 in myogenesis will be determined by transfection of the H36 gene into specific H36- developmental mutants and other appropriate cells, by using antisense oligonucleotides to inhibit its synthesis, and by blocking H36 function with monoclonal antibodies. Functional domains in the molecule will be identified by mutagenesis analysis. We will determine whether H36 and other membrane proteins known to be involved in myoblast interactions are involved in the molecular discrimination between primary and secondary myoblasts that underlies their autonomous development. These studies will resolve the functions and mechanisms that regulate H36 expression during early and terminal myogenesis and the mechanisms by which primary and secondary myoblasts interact and discriminate one another. This infommtion is essential to the fundamental understanding of cell interactions and skeletal muscle development that are needed to develop rational approaches to the selective repair and replacement of primary and secondary muscle fibers.